1. Field of the Invention
The present invention relates to image scanning devices capable of scanning images on the both surfaces of an original and, more particularly, to image scanning devices capable of reducing the white level difference between the surface and the back surface and correcting the density difference in order to prevent the occurrence of scanning density differences between the both sides.
2. Description of the Related Art
Conventionally, there have been suggested and provided various types of image scanning devices capable of concurrently scanning images on the both surfaces of an original while conveying the original. Among them, there have been suggested sheet-feeding type image scanning devices including scanning means fixedly provided at the both sides of an original conveyance path and also there have been suggested image scanning devices configured to include a flat-bed type original conveying device which enables scanning non-conveyable originals such as brochures and also include a back-surface scanning means placed within the original conveying device, as described in Japanese Patent Application Laid-open (JP-A) No. 5-83480.
A conventional image scanning device capable of scanning the both surfaces of an original will be described using FIG. 8 and FIG. 9. FIG. 8 is a main part explanation view illustrating the scanning mechanism part of a conventional image scanning device and FIG. 9 is a view illustrating an image-signal processing circuit.
The image scanning device illustrated in FIG. 8 is for scanning the both surfaces of originals 39 and includes optical scanning units 36 and 40 which are placed such that they sandwich an original conveyance path. In the figure, the lower surface of the original 36 is referred to as a surface and the upper surface thereof is referred to as the back surface. A feeding roller 38 for conveying originals 39 is used for successive scanning of originals 39 or double-surface scanning.
The optical scanning unit 36 is a scanning means for scaring the surfaces of originals 39 and is movable within a flat bed part 35. The optical scanning unit 36 can scan an original secured on a platen glass by moving and also can scan an original being conveyed at a standstill state. The optical scanning unit 36 includes a lamp 42 and a CCD 44. Further, on the flat bed part 35, there is provided a white reference plate 37 for the optical scanning unit 36.
The optical scanning unit 40 is a scanning means for scanning the back surface of an original 39 and is fixedly placed within the original conveying device. The optical scanning unit 40 includes a lamp 43 and a CCD 45. At a position facing to the optical scanning unit 40, there is placed a platen roller 41 for the optical scanning unit 40, the platen roller 41 being also used as a white reference.
During double-surface scanning operations, the optical scanning unit 36 for surface scanning moves to the white reference plate 37 and scans the white reference plate 37 for adjusting the white level thereof and then it is moved to a usual scanning position (the illustrated position) and stopped. The optical scanning unit 40 for back-surface scanning scans a white reference on the platen roller 41 for adjusting the white level thereof and then waits an original 39 being conveyed thereto.
The originals 39 are successively fed to the original conveyance path through the feeding roller 38. Then, in the respective optical scanning units 36 and 40, the originals 39 are irradiated with the lamps 42 and 43 at their to-be-scanned regions on the surface and the back surface and are scanned by the CCDs 44 and 45.
As in the image-signal processing circuit illustrated in FIG. 9, the CCDs 44 and 45 are connected to amplifiers 46 and 47, A/D conversion circuits 48 and 49, white-level correction circuits 50 and 51, and memories 52 and 53, respectively. The white levels of the white references which are first scanned by the surface scanning CCD 44 and the back-surface scanning CCD 45 are respectively stored in the white-level correcting circuits 50 and 51 and are corrected according to the ground colors of originals which are subsequently scanned. The corrected white levels are output to the A/D conversion circuits 48 and 49. The A/D conversion circuits 48 and 49 convert analog image signals (video signals) received from the CCDs 44 and 45 through the amplifiers 46 and 47 into image data with corresponding density levels, by setting the white levels supplied from the white level correcting circuits 50 and 51 to the density saturation values. Thus, images on the surface and the back surface of the original are scanned with a proper contrast and then stored in the memories 52 and 53.
As described above, in a conventional double-surface image scanning device, a surface-scanning optical scanning unit and a back-surface-scanning optical scanning unit create white levels using specific white references such as a white reference plate and a platen roller.
However, (1) in the case where the respective white references are made of different materials, there is a reflectivity difference there between. (2) Even when the respective white references are made of the same material, the individual components cause density variations. (3) Along with the conveyance of originals, the difference in the degree of contaminations between the white references is advanced. In this case, the surface white reference plate 37 is not contaminated while the platen roller 41 which is the back-surface white reference is contaminated at its surface along with conveyance of originals.
For these reasons, the white reference levels of the optical scanning units 36 and 40 may not be in agreement with each other, thereby causing density differences among scanned image data. Consequently, there has been a need for performing burdensome level adjustments. Particularly, in the case of color scanning, the reflectivity differences on the respective color constituent basis (RGB) will cause color differences in scanned images and, therefore, it has been necessary to perform, for the respective white reference members, complicated controls, such as measuring the reflectivities of the respective RGB constituents and setting the white reference levels based on the reflectivities, for the respective components.
Further, JP-A No. 4-371072 discloses a configuration which scans images on the surface and the back surface of a prepared white reference test chart and adjusts the levels such that the scanned values obtained from the surface and the back surface are in agreement with each other. However, this configuration can overcome the aforementioned problems (1) and (2), but can not address the problem (3). Further, there is a need for preparing a sheet for adjustment, thereby causing the problems of cost increases and the necessity of adjusting operations.
Further, JP-A No. 2002-290685 and JP-A No. 2002-335380 disclose configurations in which a surface-scanning optical scanning unit and a back-surface-scanning optical scanning unit scan a single white reference member to perform white level correction. However, even though a single white reference member is used for white level correction, the surface-scanning optical scanning unit and the back-surface scanning optical scanning unit scan different regions, thereby causing the problem of impossibility of proper white level correction.